1. Field of the Invention
This invention relates to heating or combustion means which utilize rabble teeth for moving and stirring the material being processed, and to a method of heating or combusting such material.
2. Description of the Prior Art
Multiple hearth furnaces such as the Herreshoff furnace are characterized by a plurality of vertically aligned hearths, or hearth chambers, down through which material being processed is moved. The temperature and atmosphere in the various hearth chambers can be controlled individually and, depending on the particular construction of the furnace, the rate of movement through it can be controlled for each hearth.
Multiple hearth furnaces of the class described may include an elongated rotatable center shaft which extends up through the center of the furnace, passing through each hearth floor. Rabble arms are secured to the center shaft and extend radially outward therefrom over each hearth floor. These rabble arms are provided with rabble teeth which extend down into the material being processed on the hearth. As the center shaft rotates, the rabble arms move over the material being processed while the rabble teeth plow through it. Depending on the angle of inclination of the rabble teeth, the material will be moved radially inwardly toward the center shaft or outwardly therefrom. Drop holes are provided in the floor of each hearth, either in toward the center shaft or out toward the furnace walls so that as the material completes its movement over the hearth it will drop down into the next lower hearth and move across this hearth in the opposite direction. Thus the material is caused to move slowly in serpentine fashion through the furnace.
The multiple hearth furnace possesses certain advantages over other solid material processing furances such as rotary kilns and the like. Thus, because these furnaces permit control of individual hearth atmospheres and temperatures, it is possible to perform very delicate operations such as regeneration of bone char and of certain granular activated carbons.
Another advantage of multiple hearth furnaces for the processing of dry solid materials lie in their ability to maintain such material in mixed condition throughout their passage through the furnace. In inclined rotary kilns, on the other hand, particles of the material being processed tend to segregate according to size, with the result that certain portions of the material have longer exposure to furnace atmospheres and temperature and consequently become processed to a different degree than other portions. The multiple hearth furnace avoids those difficulties and insures that all sizes of particles being processed receive substantially the same treatment.
Rabble teeth, as disclosed in the von Dreusche et al U.S. Pat. No. 3,402,920 have been fabricated of metal and because of the furnace atmosphere, have been subject to the action of corrosive gases. As set forth in von Dreusche U.S. Pat. No. 3,419,254, rabble teeth were constructed of ceramic material when used at very high temperature. Although the materials used for the fabrication of rabble teeth in the referred to patents have proven to be commercially desirable, more recent technological advances have opened the way for further improvements.
It is well known that when metal teeth are used in the presence of corrosive gases, deterioration may take place rapidly and therefore considerable down time for frequent replacement is necessary. The ordinarily available ceramic materials used to fabricate the rabble teeth, lack mechanical strength so that frequent replacement may also be necessary.